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The GEOSAR system consists of 406 MHz repeaters carried on board various geostationary satellites, and the associated ground facilities called GEOLUTs which process the satellite signal.

As a GEOSAR satellite remains fixed relative to the Earth, there is no Doppler effect on the received frequency and Doppler radio location positioning techniques cannot be used to locate distress beacons. To provide rescuers with beacon position information, such information must be either:

• acquired by the beacon through an internal or an external navigation receiver and encoded in the beacon message, or

• derived, with possible delays, from the LEOSAR System.

  [GEOSAR Coverage Map]

Cospas-Sarsat has demonstrated that the detection and location of 406 MHz distress beacon signals can be greatly facilitated by global monitoring based on low-altitude spacecraft in near-polar orbits. Complete, yet non continuous coverage of the Earth is achieved using simple emergency beacons operating on 406 MHz to signal a distress. The coverage is not continuous because polar orbiting satellites can only view a portion of the Earth at any given time (see figure at left). Consequently the System cannot produce distress alerts until the satellite is in a position where it can “see” the distress beacon. However, since the satellite onboard 406 MHz processor includes a memory module, the satellite is able to store distress beacon information and rebroadcast it when the satellite comes within view of a LUT, thereby providing global coverage.

As described above, a single satellite, circling the Earth around the poles, eventually views the entire Earth surface. The “orbital plane”, or path of the satellite, remains fixed, while the Earth rotates underneath it. At most, it takes only one half rotation of the Earth (i.e. 12 hours) for any location to pass under the orbital plane. With a second satellite, having an orbital plane at right angles to the first, only one quarter of a rotation is required, or 6 hours maximum. Similarly, as more satellites orbit the Earth in different planes, the waiting time is further reduced. The Cospas-Sarsat System design constellation is four satellites which provide a typical waiting time of less than one hour at mid-latitudes.

The LEOSAR system calculates the location of distress events using Doppler processing techniques. Doppler processing is based upon the principle that the frequency of the distress beacon, as “heard” by the satellite instrument, is affected by the relative velocity of the satellite with respect to the beacon. By monitoring the change of the beacon frequency of the received beacon signal and knowing the exact position of the satellite, the LUT is able to calculate the location of the beacon.

There are three types of LUTs in the Cospas-Sarsat System, each corresponding to the type of satellite constellation that they operate with: LEOLUTs for the LEOSAR system, GEOLUTs for the GEOSAR system, and MEOLUTs for the MEOSAR system.

LEOLUT GEOLUT and MEOLUT operators provide the SAR community with reliable alert and location data without restriction on its use and distribution. The Cospas-Sarsat Space Segment Providers supply LUT operators with the System data that is required to operate their LUTs. To ensure that the data provided by LUTs is reliable and can be used by the SAR community on an operational basis, Cospas-Sarsat has developed LUT performance specifications and procedures. Copies of the LUT specifications (documents C/S T.002 for LEOLUTs, C/S T.009 for GEOLUTs and C/S T.019 for MEOLUTs) and commissioning standards (documents C/S T.005 for LEOLUTs, C/S T.010 for GEOLUTs and C/S T.020 for MEOLUTs) are available for download under the “System Documents” section of the Professionals website (Pro/Documents).

 

LEOLUTs

[Map of LEOLUT Locations]

The configuration and capabilities of each LEOLUT may vary to meet the specific requirements of the participating countries, but the Cospas and Sarsat LEOSAR spacecraft downlink signal formats ensure interoperability between the various spacecraft and all LEOLUTs meeting Cospas-Sarsat specifications.

The capability of a LEOLUT is determined, for the most part, by the LEOSAR satellite channels it was designed to process. There are a possible 2 channels that may, depending upon the specific satellite being tracked, be available for processing. Some satellites support all the channels listed below, and some only support a limited set of them.

• The 406-MHz Search and Rescue Processor (SARP) satellite channel transmits received 406-MHz beacon data that has already been partially processed by the satellite to determine the identification, transmit time, and received frequency for each distress beacon transmission burst. Because of the on-board memory capability of the SARP channel, this channel provides global (yet not continuous) coverage for distress beacons that operate at 406 MHz.

• The 406-MHz Search and Rescue Repeater (SARR) channel receives 406-MHz beacon transmission bursts and immediately retransmits them on the satellite downlink. Since there is no memory associated with the repeater channel, this type of processing supports only local mode coverage (i.e., the distress beacon and the LEOLUT must be in simultaneous view of the satellite for a period of time). Furthermore, since the satellite does not process the data, all the processing is performed by the LEOLUT.

For 406-MHz signals received via their respective SARR channel, each transmission is detected and the Doppler information calculated. A beacon position is then determined using this data. The LUT is also able to provide identification information associated with the beacon.

Processing the SARP channel 2400-bps data (i.e., those generated from 406-MHz transmissions) is relatively straightforward since the Doppler frequency is measured and time-tagged on-board the spacecraft. All 406-MHz data received from the satellite memory on each pass can be processed within a few minutes of pass completion.

To maintain accurate location processing, an update of the satellite ephemeris is produced each time the LUT receives a satellite signal. The downlink carrier is monitored to provide a Doppler signal using the LUT location as a reference, or highly stable 406-MHz calibration beacons at accurately known locations are used to update the ephemeris data.

 

GEOLUTs

[Map of GEOLUT Locations]

A GEOLUT is a ground receiving station in the Cospas-Sarsat System that receives and processes 406-MHz distress beacon signals which have been relayed by a Cospas-Sarsat geostationary satellite. Due to the extremely large continuous coverage footprint provided by each geostationary satellite, GEOLUTs are able to produce near-instantaneous alerting over extremely large areas. However, due to the fact that the satellite remains stationary with respect to distress beacons, GEOLUTs are not able to determine beacon locations using Doppler processing techniques. In view of this, 406-MHz beacons with location protocols allow for the encoding of GNSS position data in the transmitted 406-MHz message, thus providing for quasi-real-time alerting with position information via the GEOSAR system.

The “GEOLUT Availability Table” provides an indication of which GEOSAR satellite is tracked by which specific GEOLUT (Pro/System/System Monitoring/Availability Tables (QMS)).

 

MEOLUTs

[map of MEOLUT Locations] to come
[photo of a MEOLUT] to come

A MEOSAR Local User Terminal (MEOLUT) is a ground receiving station in the Cospas-Sarsat MEOSAR system that detects, characterizes and locates 406-MHz beacons, and forwards the beacon distress alert and location data to its associated Cospas-Sarsat Mission Control Centre (MCC).

The MEOLUT simultaneously tracks several medium earth orbiting (MEO) satellites of the BDS, Galileo, GPS and Glonass constellations that embark Search and Rescue (SAR) repeaters in addition to their primary GNSS payloads. The MEOLUT receives and processes the beacon signals relayed by those satellites and measures the received frequency and time of the beacon bursts. The MEOLUT calculates the uplink frequency of arrival (FOA) and time of arrival (TOA) of the detected beacon bursts at the satellite for each satellite channel. If the beacon burst is received from at least three MEOSAR satellites, the MEOLUT then calculates an unambiguous location for the beacon from the uplink TOA and FOA data. This methodology to determine beacon locations independently from GNSS signals is referred to as the Frequency Difference of Arrival/Time Difference of Arrival (FDOA/TDOA) location method. The MEOLUT can improve the accuracy of the beacon location over the first burst by combining data from subsequent bursts as they are received.

In addition, a MEOLUT may be exchanging data with other MEOLUTs, which allows increasing the number of TOA/FOA measurements used to locate beacons, thus increasing the beacon location accuracy and extending the coverage of the “networked” MEOLUTs.

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The use of special-purpose radiobeacons, either manually or automatically activated by an aircraft crash or maritime distress situation, reduces the time required to alert the appropriate authorities and for final location of the distress site by the rescue team

IMO Requirements

The 1988 amendment to the Convention for the Safety of Life at Sea, 1974 (SOLAS Convention) establishing the Global Maritime Distress and Safety System (GMDSS) mandates that ships of 300 tons and over carry a Satellite Emergency Position Indicating Radio Beacon (Satellite EPIRB). This carriage requirement became effective on 1 August 1993. Cospas-Sarsat compatible 406 MHz EPIRBs satisfy the alerting requirement of the GMDSS.

ICAO Requirements

The International Civil Aviation Organization (ICAO) recommends that, from July 2008, all aircraft under the jurisdiction of the ICAO Convention carry an Emergency Locator Transmitter (ELT) operating on the frequency 406 MHz for compatibility with the Cospas-Sarsat System, and on 121.5 MHz for “homing” purposes.

National Requirements

Various national requirements also exist for the carriage of ELTs/EPIRBs on various types of craft not otherwise subject to international conventions, and some countries have authorised the use of 406 MHz Personal Locator Beacons (PLBs) on land, in remote or rugged areas.

A large number of 121.5 MHz beacons are installed on board light aircraft and carried on board pleasure craft, either as a result of voluntary fitting or in response to specific national carriage requirements. These users should consider replacing their 121.5 MHz beacons with 406 MHz beacons as the satellite processing of 121.5 MHz emission was terminated on 1 February 2009.